The present invention relates to a method and system for testing the bending resistance and damping of individual gear teeth made of steel in order to determine the presence of defects. In particular it relates to such methods and systems wherein the sudden application of a force causes the tooth to undergo free flexural vibrations.
In order to judge the condition of highly-loaded gears at the beginning and during their operating life, a non-destructive testing procedure is required in order to detect defects in the material, small cracks at the base of the tooth and the beginning of defects resulting from metal fatigue at the surface of the tooth, before damage or complete destruction can occur.
For objects which have an audible characteristic vibratory frequency, a subjective testing of such errors is a well-known procedure. The procedure consists of transferring a sudden pulse of energy to the object by an impact and then deriving information concerning possible defects from the resulting vibrations and, in particularly, from the frequency spectrum (tone) and the attenuation of the oscillations (damping). This simple method cannot be applied to the testing of individual gear teeth for the following reasons:
The frequency of the characteristic flexural vibrations of a tooth are in the ultrasonic region and therefore not audible; and the time during which the force is applied, for example if the force of a hammer is applied to a tooth, comprises a multiple of the period of the characteristic vibratory frequency of a tooth, so that the applied energy cannot be efficiently converted into vibratory energy. It is required that the time duration of application of the force to the tooth be at the most equal to approximately one-half of the characteristic vibratory cycle of the tooth.
Objective methods of determining the frequency and damping of the exponentially decreasing vibration or oscillation in response to a transient impact are known which are utilized when the accuracy requirement and/or the frequency do not allow a subjective evaluation. These methods and system utilize an electrical sensor which furnishes an electrical signal corresponding to the deformation of a freely vibrating system to an electronic evaluation circuit; the force is, however, still applied by mechanical means and for an impact duration which is too long for use in the evaluation of gear teeth.
In order to be able to carry out a damping measurement in accordance with the decay of vibrations in those cases when the characteristic frequency or vibration of the test sample, as for example the gear tooth, lies in the ultrasonic region, a method and system is known and published in the journal Messen and Prufen/Automatik, 1973, Heft July/August S. 478 and Heft September S. 575) wherein the test sample is continuously energized at a frequency corresponding to its characteristic frequency of vibration and the energy supply is then abruptly cut off. Free, approximately exponentially decreasing oscillations then occur which can be readily evaluated. The transfer of vibratory energy to the test sample may take place by means of forces created in a magnetic field which is generated by an alternating circulation induced in a coil, the coil being connected as part of an oscillator tuned to the characteristic frequency of the test sample. However, this measuring process is of course made very difficult because the characteristic frequency must first be known. Further, the resonant increase of the vibration varies greatly as a function of the damping and requires that, for each tooth, an adjustment of the oscillator output must be made if a constant sensitivity is to be maintained.